1. Field of the Invention
The invention relates in general to solid deposition modeling, and in particular to a method and apparatus for providing a planar flash to cure layers of a build material to form a three-dimensional object. The flash curing system substantially eliminates the problem of oxygen inhibition encountered when attempting to cure thin layers exposed to air.
2. Description of the Prior Art
Recently, several new technologies have been developed for the rapid creation of models, prototypes, and parts for limited run manufacturing. These new technologies are generally called Solid Freeform Fabrication techniques, and are herein referred to as “SFF.” Some SFF techniques include stereolithography, selective deposition modeling, laminated object manufacturing, selective phase area deposition, multi-phase jet solidification, ballistic particle manufacturing, fused deposition modeling, particle deposition, laser sintering, and the like. Generally in SFF techniques, complex parts are produced from a modeling material in an additive fashion as opposed to conventional fabrication techniques, which are generally subtractive in nature.
In most SFF techniques, structures are formed in a layer by layer manner by solidifying or curing successive layers of a build material. For example, in stereolithography a tightly focused beam of energy, typically in the ultraviolet radiation band, is scanned across a layer of a liquid photopolymer resin to selectively cure the resin to form a structure. In Selective Deposition Modeling, herein referred to as “SDM,” a build material is typically jetted or dropped in discrete droplets, or extruded through a nozzle, in order to solidify on contact with a build platform or previous layer of solidified material in order to build up a three-dimensional object in a layerwise fashion. Other synonymous names for SDM which are used in this industry are solid object imaging, solid object modeling, fused deposition modeling, selective phase area deposition, multi-phase jet modeling, three-dimensional printing, thermal stereolithography, selective phase area deposition, ballistic particle manufacturing, fused deposition modeling, and the like. Ballistic particle manufacturing is disclosed in, for example, U.S. Pat. No. 5,216,616 to Masters. Fused deposition modeling is disclosed in, for example, U.S. Pat. No. 5,340,433 to Crump. Three-dimensional printing is disclosed in, for example, U.S. Pat. No. 5,204,055 to Sachs et al. Often a thermoplastic material having a low-melting point is used as the solid modeling material in SDM, which is delivered through a jetting system such as an extruder or print head. One type of SDM process which extrudes a thermoplastic material is described in, for example, U.S. Pat. No. 5,866,058 to Batchelder et al. One type of SDM process utilizing ink jet print heads is described in, for example, U.S. Pat. No. 5,555,176 to Menhennett et al.
Recently, there has developed an interest in utilizing curable materials in SDM. One of the first suggestions of using a radiation curable build material in SDM is found in U.S. Pat. No. 5,136,515 to Helinski, wherein it is proposed to selectively dispense a UV curable build material in an SDM system. Some of the first UV curable material formulations proposed for use in SDM systems are found in Appendix A of International Patent Publication No. WO 97/11837, where three reactive material compositions are provided. More recent teachings of using curable materials in various selective deposition modeling systems are provided in U.S. Pat. No. 6,259,962 to Gothait; U.S. Pat. No. 5,380,769 to Titterington et al; U.S. Pat. Nos. 6,133,355 and 5,855,836 to Leyden et al; U.S. Pat. App. Pub. No. US 2002/0016386 A1; and International Publication Numbers WO 01/26023, WO 00/11092, and WO 01/68375.
These curable materials generally contain photoinitiators and photopolymers which, when exposed to ultraviolet radiation (UV), begin to cross-link and solidify. Often these curable materials contain non-curable components, which enable the materials to solidify after being dispensed prior to being cured. This is generally needed so that the selectively dispensed material will maintain its shape before being exposed to ultraviolet radiation.
For SDM systems that selectively dispense curable materials, it is desirable to initiate curing of the dispensed material by a flood exposure to UV radiation. However, developing a flood UV exposure system that effectively initiates curing in these materials proven problematic. When the photoinitiators in the thin layers are excited by exposure to UV radiation, they release free radicals that are intended to react with the photopolymers and initiate the polymerization (curing) process. Because of the wide area of exposure of these thin layers to the atmosphere, the free radicals tend to react with the oxygen in the atmosphere instead of reacting with the photopolymers to initiate curing. This cure-hindering phenomena is known as oxygen inhibition, which can undesirably reduce or prevent the polymerization process from occurring. Oxygen inhibition is effectively non-existent in stereolithography since the tightly focused beam of UV radiation triggers a large instantaneous release of free radicals over a small region. The region is so small the free radicals lack the opportunity to react with the oxygen in the atmosphere. However, oxygen inhibition is a significant problem in SDM applications where a broad planar flood exposure is desired to initiate the curing process. Although this phenomena can be overcome by submersing the SDM build environment in an inert gas, providing such a system only adds additional complexity and expense to an SDM system that is to be avoided.
Most UV lamps used in curing photopolymers that provide a planar exposure of UV radiation are typically mercury-halide lamps, metal halide lamps, or mercury-xenon lamps. These lamps are continuous running lamps that generate high levels of heat in order to produce the levels of UV radiation necessary to trigger polymerization. Undesirably, the high levels of heat generated by these lamps pose significant problems in SDM. For instance, the heat generated by these lamps can thermally initiate curing of the material in the SDM dispensing device or material delivery system rendering the apparatus inoperable. Alternatively, the heat may also prevent the dispensed material from solidifying prior to being exposed to UV radiation. Undesirably, if these continuously emitting lamps are to be used, the high levels of heat they generate may require special active cooling systems to be incorporated into the system to make the system operable. Not only is the amount of power consumed by these lamps to maintain the emission substantial, but they also have long warm up times which necessitates that they be operated continuously. Thus, they typically require some sort of mechanical shutter system in order to control the duration of the exposure in SDM systems while the lamps are operated continuously. Further, experiments with continuous UV emitting lamps not only demonstrated that they consume significant amounts of power, typically around 1500 Watts, but also that many curable formulations would not cure due to oxygen inhibition.
Recently there has come available flash curing systems which generate high peak power pulses of ultraviolet radiation for curing. Although these commercially available systems are capable of overcoming the problem of oxygen inhibition, they are generally not practical for use in SDM. For example, commercially available power supplies for use in charging these pulse systems have large input power requirements, often around 40 kilowatts or more. These power supplies, initially designed to power laser systems, need more than about 1000 watts of input power to operate, and typically require a line voltage of 240 VAC or greater. Thus, these flash curing systems operate at power levels that are too high to be useful in SDM systems. For SDM applications a lower power consumption flash curing system is needed, but not available.
Thus, there is a need to develop an inexpensive, low power, flash curing system for SDM capable of initiating polymerization of selectively dispensed curable materials without detrimentally effecting the layer-by-layer SDM build process. These and other difficulties of the prior art have been overcome according to the present invention.